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Re/Os cosmochronometer:
measurements of relevant Os
cross sections
Marita Mosconi1, Alberto Mengoni2, Michael Heil1, Ralf Plag1, Franz Käppeler1,
Kaori Fuji5, Gaelle Aerts3, Rita Terlizzi4 and the n_TOF collaboration
•186Os, 187Os and 188Os (n,) cross sections at the n_TOF facility
operating at CERN
• Inelastic scattering cross section of
Graaf
187Os
at 3.7 MV FZK Van de
Forschungszentrum Karlsruhe GmbH (FZK), Institut für Kernphysik, Germany
CERN, Geneva, Switzerland; IAEA, Vienna, Austria
3 CEA/Saclay, Gif-sur-Yvette, France
4 INFN Bari, Italy
5 INFN Trieste,Italy
1
2
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
1
The Re/Os cosmochronometer
s-process local approximation: ANA= A-1NA-1
s olar
Nrad

N

187
187
Os
Os
σ 186 O s (n, )
σ 187 O s (n, )
es s
Ns186proc
Os
At stellar temperatures ,187Os low-lying excited states are populated:
theoretical calculations are needed to evaluate the stellar capture rate
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
2
187Os
7/2-
100.0 keV
5/2-
75.0 keV
3/2-
74.3 keV
3/21/2-
stellar rate
9.75 keV
Theoretical calculations can use as input
experimental results:
187Os
Thermal population
at 30 keV:
P(gs)
= 33%
P(1st)
= 47%
P(all others) = 20%
the measurement of the inelastic scattering
cross section for the first excited level allows
to define the competition by the scattering
channels under stellar conditions
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
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Clock determination
•
temperature dependence of t1/2(187Re)
(Bosch et al., Phys.Rev.Lett 77 (1996))
• 186Os, 187Os(n,) cross sections from
Nuclear
0.1 keV to 500 keV
quantities
• evaluation of stellar cross sections
including the effect of excited states
(inelastic scattering cross section on
187Os )
Astrophysics
• astration
• galactic history of the
Santa Tecla, 2-9 October 2005
187Re
Marita Mosconi,FZK
enrichment
4
Measurements of the (n,)
cross section at n_TOF
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
5
Experimental setup at n_TOF
Spallation neutron source: 20GeV
protons on a lead target providing a
wide range of neutron energies
(2003)
-ray detection: C6D6 scintillators
Sample
changer
C6D6
C6D6
Neutron beam
Pulse height weighting technique
Neutron flux monitor: Silicon
detectors viewing a thin 6LiF foil
Santa Tecla, 2-9 October 2005
Correction of the -response by
weighting function to make the
detector efficiency proportional to
-ray energy
Marita Mosconi,FZK
6
Samples measured and yield
•
•
•
186Os
(2 g, 79 %)
187Os (2 g, 70 %)
188Os (2 g, 95 %)
• Al can
 environmental background
• natPb (2 g)
 in-beam gamma background
• natC (0.5 g)
 neutron scattering
background
• 197Au (1.2g)
 flux normalization
(using Ratynski and Macklin
cross section measurements
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
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Preliminary results
Maxwellian averaged cross sections at 30 keV
n_TOF
Winters
Browne
186Os
409±20
418±16
438±30
187Os
931±47
874±28
919±28
188Os
298±15
401±15
395±24
(mb)
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
(mb)
(mb)
8
Scattering experiment at FZK
Van de Graaff
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
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Experimental method
• identification of the inelastic scattering component by energy
separation
• measure the cross section as closer as possible to the energies
relevant for s-process
• Time-of-Flight technique
• neutron beam with a narrow energy distribution (FWHM≤10 keV)
• use of 7Li(p,n)7Be reaction:
θ=60°
θ=45°
θ=30°
1. narrow neutron distributions for
En≥ 60 keV
θ=25°
2. Gaussian neutron energy
distribution centered at 30 keV
θ=15°
FWHM≤10
θ=0°
|  | arcsin
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
Ep-Eth≤0.3 keV
M7 BeM7 Li Ep  Eth
MpMn
Ep
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Experimental setup and DAQ
Main drawback of 7Li(p,n)7Be
at threshold: small yield
Neutron
monitor
• improved neutron detection
efficiency by use of KG2
(NE912)
• background subtraction by
pulse shape analysis
6Li-glass
Beam unstable due to proton
source instabilities:
computer monitoring of the
neutron energy distribution
PMT
Samples measured
•
187Os
• 188Os
pure elastic component
• Empty canning
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
background
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Data
Two week of data taking
with proton current 3-6 μA
Data analysis
• Runs with similar FWHM
extracted
• Parameter of the
scattering component
extracted
• Fit of the 187Os scattering
distribution thought as
made by two separated
components
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
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Summary
186,7,8Os(n,)
cross sections:
• background subtraction concluded
• resonances analysis in progress
• isotopic correction to be improved using the results of the
previous item
187Os(n,n’)
cross sections:
• data taken for neutron distribution centered at 30
keV and with FWHM from 7 to 10 keV
• analysis in progress
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
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The End
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
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Derivation of the cross sections in the
unresolved region
1.
-response corrected by weighting functions
2.
background subtraction
188Os
with filters
Estimated level of
background
• neutron scattering background evaluated
using the C measurement (environmental
background corrected) rescaled for the
ratio of the neutron scattering cross
sections of carbon and of the sample
analyzed
• shape of the -background determined
using Pb measurement after subtraction of
it own neutron scattering background and
environmental background
• level of the -background determined by
GEANT3.21 simulations
Normalisation
0.992
Background
0.000
Au 4.9 eV
resonance
SAMMY fit
3. neutron flux determined by Au count rate
(using Ratynski and Macklin cross section
measurements)
4.
isotopic correction
Santa Tecla, 2-9 October 2005
Marita Mosconi,FZK
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